1. Field of the Invention
This invention relates generally to portable power tools, and in particular to a drive shaft assembly for transmitting torque from a power unit to a rotary attachment such as a monofilament line trimmer.
2. Description of the Prior Art
It is well known in the prior art to provide an internal combustion engine for driving portable power tools such as flexible line trimmers, lawn edgers, brush cutters and the like. In the typical prior art arrangement, the rotary head of the power tool is connected by means of a flexible drive shaft enclosed within a long tubular housing and is connected in driving relation at the opposite end to the internal combustion engine. The tubular housing is relatively long as compared to the length of the power handle and is slightly curved at its end to maintain the cutter head at a preferred cutting angle with respect to the longitudinal axis of the engine housing. This arrangement permits the assembly to be carried and manipulated comfortably by an operator in an erect standing or walking position. Because the lower end of the drive shaft transitions through an arc to produce the desired angular relationship, a flexible drive shaft must be used to negotiate the arc for efficiently transmitting power from the engine to the rotary head.
A problem inherent in this curved drive shaft arrangement is that of "whipping" of the flexible drive shaft against its tubular housing enclosure which causes premature wear and failure. This problem has been alleviated somewhat by such techniques as enclosing the flexible drive shaft within a steel sheath, and by supporting the flexible drive shaft at a number of axially spaced stations throughout the tubular housing by annular bushings. Neither approach has been completely satisfactory for a number of reasons. For example, the metal liner or sheath imposes a substantial friction load which causes premature failure because of thermal cycling and abrasive wear against the metal liner surface. Additionally, this arrangement is objectionable because of the noise produced by the metal-to-metal contact as the flexible drive shaft rotates. The spaced bushing assembly represents an improvement over the metal sheath insofar as noise level and wear are concerned; however, the initial placement of the bushings at the proper locations within the curved, tubular housing is a difficult operation, and the insertion of a replacement flexible drive shaft in the curved tubular housing is extremely difficult and requires the removal and replacement of the bushings within the curved section. Additionally, because of the spacing between bushings throughout the curved section, a substantial amount of "whipping" occurs in the curved section at high speed operation.